1. Technical Field
The present invention relates to a device for driving a rotary solenoid.
2. BACKGROUND ART
The suspension of an automobile is adjusted by using, for example, a rotary solenoid which works to change the suspension characteristics into a soft mode and a hard mode, and such a rotary solenoid is driven by an H-bridge circuit as shown in FIG. 3.
That is, one end of the rotary solenoid RS is connected between a first transistor Tr1 and a second transistor Tr2 of a circuit that connects the power source V.sub.B ground via first and second transistors Tr1, Tr2. The other end of the rotary solenoid RS is connected between a third transistor Tr3 and a fourth transistor Tr4 of a circuit that connects the power source V.sub.B to ground via third and fourth transistors Tr3, Tr4. The first to fourth transistors Tr1 through Tr4 are rendered conductive or non-conductive in response to two signals A, B from a CPU (not shown in the figures), in order to turn the rotary solenoid RS forwardly, reversely or to hold it.
As the two signals, the CPU sends a forward instruction signal (switch-into hard signal) A and a reverse instruction signal (switch-into soft signal) B, as shown in FIG. 4.
Usually, when both the forward instruction signal A and the reverse instruction signal B have the L level, the forward instruction signal A which has the L level renders the first transistor Tr1 conductive and the second transistor Tr2 non-conductive. Further, the reverse instruction signal B which has the L level, renders the third transistor Tr3 conductive and the fourth transistor Tr4 non-conductive. In this case, therefore, the electric current to the rotary solenoid RS is interrupted, and the rotary solenoid RS is maintained in the holding condition.
To forwardly rotate, the forward instruction signal A only is allowed to have the H level. Then, the first transistor Tr1 is rendered non-conductive and the second transistor Tr2 is rendered conductive. Here, since the reverse instruction signal B remains at the L level, the third transistor Tr3 is rendered conductive and the fourth transistor Tr4 is rendered non-conductive. In this case, therefore, a current circuit in a direction of the arrow F in FIG. 3 is established from the power source V.sub.B to the third transistor Tr3, to the rotary solenoid RS, and to the second transistor Tr2, and the rotary solenoid RS rotates in the forward direction.
To reversely rotate, the reverse instruction signal B only is allowed to have the H level. Then, the third transistor Tr3 is rendered non-conductive and the fourth transistor Tr4 is rendered conductive. Moreover, since the forward instruction signal A remains at the L level, the first transistor Tr1 is rendered conductive and the second transistor Tr2 is rendered non-conductive. In this case, therefore, a current circuit in a direction of the arrow R in FIG. 3 is established from the power source V.sub.B to the first transistor Tr1, to the rotary solenoid RS, and to the fourth transistor Tr4, and the rotary solenoid RS rotates in the reverse direction.
According to the above-mentioned conventional device for driving the rotary solenoid, however, there momentarily takes place a short-circuited condition in which both the first transistor Tr1 and the second transistor Tr2 are rendered conductive at a moment when the forward instruction signal A rises or breaks, due to a delay in the response of the circuit. In the worst case, therefore, the transistors may be burnt out.
At a moment when the reverse instruction signal B rises or breaks, furthermore, the third transistor Tr3 and the fourth transistor Tr4 may develop similar problems. If the CPU operates abnormally, furthermore, the electric current to the rotary solenoid is interrupted to maintain reliability and safety.
An object of the present invention, therefore, is to provide a device for driving a rotary solenoid which will not develop such problems.